1. Field of the Invention
The present invention relates to an Overhear-based transmitting control system for improving performance of 802.11 WLANs. The main idea of this scheme is that when the transmission of a node fails, one of neighbor nodes with better channel condition transmits the lost frame as well as the own data using MPDU aggregation scheme. To do so, every node should manage overhear table to buffer the transmitted packets that is not yet acknowledged. We also present algorithms to retransmit lost packets, to maintain the overhear table and to compensate for the retransmission of packets of other nodes.
2. Description of the Related Art
IEEE 802.11 standard supports two different MAC protocols. One is DCF and the other is PCF. In DCF, when stations face the failure of transmission, they retransmit the collided packets according to binary exponential backoff rules. Backoff rule is that each station delays to transmit the packets for a time period which is chosen uniformly from the range of 0 to its contention window size (CW). After each successful data transmission, the window size is set to CWmin, which denotes the prespecified minimum contention window. This makes the delay of transmission by each station and the throughput performance of the 802.11 WLAN poor according to the number of retransmission increases. Because retransmission of failed data allows other stations to lose their channel access chances and increases the number of MAC/PHY overheads.
To exploit the improvement using physical layer multirate capability, researchers have proposed various protocols. At the network layer, some channel state aware routing schemes have been studied to improve the end-to-end throughput by taking into account the channel condition as one of the route selection metrics. However, due to the long latency of route updates and the high control overhead, these schemes cannot quickly react to dynamic channel condition and can not achieve high bandwidth utilization. At the MAC layer, many schemes have been proposed to exploit the multirate capability. The basic idea of these schemes is to let the sender select a proper transmission rate according to the history of the successful transmissions or to let the receiver sense the channel condition before the transmission and notify the sender via a control packet (e.g., the clear-to-send (CTS) packet). However, these schemes only utilize the data rate of the direct link between the sender and the receiver. In many cases, data may be delivered much faster through multiple links that have high channel quality than through the direct link with the same transmission rate.
Hao Zho and Shihong Zou designed a novel relay-enabled DCF protocol, called rDCF, to exploit the physical layer multirate capability. According to the channel condition, data can be transmitted with different rates and some data packets may be delivered faster through a relay node than through the direct link if the direct link has low quality and low rate. The basic protocol of rDCF is proposed to help the sender, the relay node, and the receiver coordinate to decide what data rate to use and whether to use a relay node. However, rDCF are insufficient to decrease the number of MAC/PHY overheads because it did not consider the aggregated data scheme. And it does not consider the throughput gain of the relay-enabled DCF protocol when every station has the same physical layer rate. Finally, rDCF is possible in assuming whenever every station can do the triangle handshake with any stations.